A conventional CMOS switch comprises one or more CMOS transistors, each with its bulk (e.g., substrate or well) connected to one of the power supply rails (i.e., Vdd or Vss). For example, a single N-type CMOS (NMOS) transistor, with its drain connected to the input node Vin, its source connected to the output node Vout, its gate connected to receive a switch-control signal, and its bulk connected to Vss, can function as a CMOS switch that selectively presents an input voltage appearing at node Vin as an output voltage at node Vout, where the value of the switch-control signal applied to the transistor gate determines whether the switch passes or holds off the input signal.
Another example of a conventional CMOS switch is formed from an NMOS transistor connected in parallel to a P-type CMOS (i.e., PMOS) transistor, where the NMOS transistor is configured as before, and the PMOS transistor has its source connected to node Vin, its drain connected to node Vout, its gate connected to receive an inverted version of the switch-control signal, and its bulk connected to Vdd.
The ranges of voltages that can be applied to such conventional CMOS switches are often limited due to finite N-channel and/or P-channel thresholds. In some situations, the allowable input range spans only a portion of the available supply voltage range (e.g., Vdd-Vss). Moreover, any voltage beyond the supply voltage range is usually not allowed, since it may interfere with the proper operation of the switch in its open (i.e., off) mode.
To accommodate an input voltage range beyond the supply voltage range, some prior-art implementations rely on a boosted supply. This more-positive and/or more-negative supply is often locally generated and used instead of the PC board power supply, in effect operating the switch from a new power supply that now includes the desired expanded range.
Another prior-art implementation relies on attenuation of all input voltages to ensure that the input voltage levels remain within the allowable range.